This invention relates to a solid bowl centrifuge. More particularly, this invention relates to a solid bowl centrifuge with a beach area between a pool section and a cake discharge opening.
Some slurries containing granular solids, such as plastics or polymers, are mechanically dewatered by a solid bowl centrifuge to the lowest possible moisture before being sent for thermal drying. In the solid bowl centrifuge, as illustrated in FIG. 1A, the solids in the feed slurry rapidly settle out to a cylindrical wall 10 of the centrifuge bowl 12, forming a granular cake 14c. The cake, by a differential rotation between a screw-type conveyor 16 and the bowl 12, is transported from a cylinder section of the bowl to a conical section 18 thereof. Also, an annular slurry pool 20 forms in the bowl. The cake 14 is moved through from below the pool to above the pool in the conical section 18 which is commonly referred to as the dry beach. There, inasmuch as the cake is outside the pool of liquid, the cake can further dewater, with liquid draining through the cake as a result of the centrifugal gravity G. The drained water passes through a gap 22 formed between the conveyor blade tip 24 and the inner surface 26 of the conical bowl wall or section 18. A pressure face 28 of the conveyor blade 30 (see FIG. 1A) and the inner surface 26 of the conical section 18 of the bowl 12, together with the blade tip gap 22, form a funnel through which the liquid filtrate passes under the influence of centrifugal gravity G. After the liquid flows through this gap 22, the water runs down along a helical space adjacent to a trailing face 32 of conveyor blade 30.
This drainage scenario is possible at low solids throughput provided that the cake profile does not bridge across the channel 34 (FIG. 1A) formed between adjacent screw conveyor flights or blades 30. The cake surface has an angle of repose a which is typically 15.degree.-45.degree. with respect to the axis 18a of the machine. The cake profile formed depends on the solids throughput, the beach angle .beta. and the angle of repose a which in turn is a function of the physical properties of the cake as well as the moisture content. The larger the angle of repose .alpha., the less likely the cake will bridge across adjacent flights or wraps of the conveyor blade 30.
At high solids throughput, the cake thickness 14a and width 14b both increase, as illustrated in FIG. 1B. The cake width eventually increases above the pitch (distance between adjacent flight discounting the blade thickness) to span across the entire helical channel 34. The helical space which would allow the liquid to run down the conical beach section 18 to the pool 20 is blocked or filled up by the cake. In this case, the "expressed" liquid 22a from the cake has to permeate through the relatively impervious cake back to the pool 20 along the conical beach section 18 by a component of centrifugal force acting along the beach. In between successive wraps of the helical channel 34, the liquid has to run through the gap 22 between the blade tip and the bowl. The controlling factors on draining the liquid down the beach 18 are the G-force, beach angle, and cake permeability which depends on the particle average size and distribution. The drainage rate is therefore much reduced as compared to the case at low solids throughput where the helix space behind the trailing face of the blade is not blocked by the cake and available for drainage. Consequently, most of the expressed liquid, instead of draining back to the pool 20, is carried along with the cake towards the cake discharge 22b, rendering the cake very wet.
FIG. 2 is a graph illustrating the variation of cake moisture as a function of cake throughput based on dry solids mass rate. FIG. 2 graphically indicates the result of the above-described drainage process. At low solids throughput, to the left of a critical point CP, the cake moisture increases only slightly with increasing throughput due to increase in cake thickness which gives higher resistance to liquid drainage within the cake. This slight increase of moisture with throughput ceases to hold after a certain throughput, corresponding to critical point CP. A further increase in cake throughput rate beyond the critical point CP triggers a much higher increase in cake moisture, as indicated by the graph line to the right of the critical point CP in FIG. 2. Above this critical rate, the cake width is large enough to span the entire channel 34, blocking liquid drainage. Typically, for cake with fine granular polymeric solids, the liquid does not effectively drain down the slope through the cake bed despite the centrifugal gravity and the steep beach. Therefore, any expressed liquid from the cake is carried with the cake to the discharge opening, thus yielding wet cake. The cake can reach 100% liquid saturation (i.e., void volume within cake all filled with liquid) or high moisture content, resulting in a steep rise in moisture with increasing rate. Short of increasing the pitch and/or beach angle, both of which has other negative impacts on process performance and mechanical condition of the machine, the present disclosure provides two innovative designs in which the beach angle end the pitch need not be compromised.